Rear or side discharge ride-on. Which is the best?

This depends entirely on the job you want to do.

Rear Discharge and collection

Rear discharge machines are designed with collection in mind, which is why they almost always come with a grass collector included. 

Most rear discharge machines use the airflow created by the spinning blades to carry the grass clippings up and back into the collector. 

This is effective in both wet and dry conditions as the air helps to separate the blades and keeps the collection efficient. 

On some top end ride-ons the rear collection is aided by a powered brush, which has it’s own PTO (power take-off) and sweeps the grass cuttings and other debris like twigs and leaves back, up and into the rear collector.

You can, of course, discharge without collection but that is not very common with rear discharge machines. 

Side Discharge

Side discharge machines are very good at handling longer grass on larger areas where collecting is not ideal, and are therefore not designed for collecting grass in the same way as rear collect machines.

You can collect with a side-discharge deck,  but the distance from the discharge to the collector is longer and so cutting and collecting in wet conditions is nowhere near as effective with a side deck and clogging can occur. Side discharge drive-ins seldom, if ever, come with collectors included.